1. Field of the Invention
The present invention relates generally to ink-jet printing and, more specifically to ink-jet pen alignment using test pattern analysis in a hard copy apparatus' self-test mode.
2. Description of Related Art
The art of ink-jet technology is relatively well developed. Commercial products such as computer printers, graphics plotters, copiers, and facsimile machines employ ink-jet technology for producing hard copy. The basics of this technology are disclosed, for example, in various articles in the Hewlett-Packard Journal, see e.g., Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No.1 (February 1994) editions. Ink-jet devices are also described by W. J. Lloyd and H. T. Taub in Output Hardcopy [sic] Devices, chapter 13 (Ed. R. C. Durbeck and S. Sherr, Academic Press, San Diego, 1988).
An ink-jet pen includes a printhead which consists of a number of columns of ink nozzles. The nozzles are employed by printhead drop generating devices (generally thermal, piezoelectric, or wave propagation types) to fire ink droplets that are used to create a printed dots on an adjacently positioned print media as the pen is scanned across the media (for convenience of description, all print media is generically referred to as "paper" hereinafter). Generally, the pen scanning axis is referred to as the x-axis, the print media transport axis is referred to as the y-axis, and the ink drop firing direction from pen to paper is referred to as the z-axis. Within the columns of nozzles, groups of nozzles, called primitives are used to form nozzle arrays grouped by ink color, e.g., four primitives within a column for cyan, yellow, magenta, or black ink ("CYMK"). A given nozzle of the printhead is used to address a given vertical column position on the paper, referred to as a picture element, or "pixel," where each nozzle-fired drop may be only a few picoliters (10.sup.-12 liter) in volume and the resultant ink dot only 1/600th-inch. Horizontal positions on the paper are addressed by repeatedly firing a given nozzle as the pen is rapidly scanned across the adjacent paper. Thus, a single sweep scan of the pen can print a swath of dots generally equivalent to the nozzle column height. Dot matrix manipulation is used to form alphanumeric characters, graphical images, and photographic reproductions from the ink drops. The print media is stepped in the y-axis to permit a series of scans, the printed swaths combining to form text or images.
In general, ink-jet hard copy apparatus are provided with two to four pens; either a set of three single color pens, or a single pen with three colorant reservoirs and at least three primitives, and a black ink pen. It is also known to print composite black using color ink. Static pen, and hence printhead nozzle alignment, is a function of the mechanical tolerances of the scanning carriage mounts for the individual pens. Moreover, ink-jet writing systems with reciprocating carriages typically have inherent dot placement errors associated with the dynamics of carriage motion. Such errors are usually associated with vibrations and therefore are cyclical in nature. If printing with a constant carriage velocity, these errors will manifest themselves on the paper at regular spatial pitches across the width of the page. Thus, among other factors, the pitch of the error will be a function of carriage velocity.
One method for determining and correcting nozzle-firing algorithms for pen alignment error parameters is where a hard copy apparatus prints a test pattern and uses the test pattern to determine the pen alignment error parameters. [Note that nozzle firing manipulation via computerized program routines, "algorithms," is a complex art in and of itself. While knowledge in that field is helpful, it is not essential to an understanding of the present invention which relates to printing error parameter derivations subsequently used by such nozzle firing algorithms.] Many such systems require the end user to inspect a variety of patterns visually and to select the pattern, and hence the hard copy apparatus settings, which are most appealing to that individual.
In U.S. Pat. No. 5,250,956, Haselby et al. use a test pattern for print cartridge bidirectional alignment in the carriage scanning axis; in U.S. Pat. No. 5,297,017, Haselby uses a test pattern for print cartridge alignment in the paper feed axis.
In U.S. Pat. No. 5,262,797, Boeller et al. disclose a standard pen plotter related method of monitoring and controlling quality of pen markings on plotting media in which an actual line plot is optically sensed across a selected point to make a comparison with a test line.
In U.S. Pat. No. 5,289,208, Haselby discloses an automatic print cartridge alignment sensor system.
In U.S. Pat. No. 5,448,269, Beauchamp et al. use a test pattern for multiple ink-jet cartridge alignment for bidirectional printing.
In U.S. Pat. No. 5,451,990, Sorenson et al. use specified test patterns as a reference for aligning multiple ink-jet cartridges.
In U.S. Pat. No. 5,600,350, Cobbs et al. teach multiple ink-jet print cartridge alignment by scanning a reference pattern and sampling the same with reference to a position encoder.
[Each patent listed above is assigned to the common assignee of the present invention. It is also known to use test patterns for testing and clearing of nozzles, testing ink quality, and for color correction; those functions are beyond the scope of the present invention and require no further explanation for an understanding of the present invention.]
Generally, large format ink-jet plotters use the strategy of using one block of nozzles from one column on one printhead as a reference. All other nozzles on every printhead are then aligned relative to this reference block.
There remains a need in the state-of-the-art for more accurate methodologies for aligning ink-jet printheads. There remains a need for automatic alignment of ink-jet printheads, that is, without the need for reliance on the user's visual acuity. There remains a need for techniques for avoiding carriage-induced dynamic errors during automated alignment of ink-jet printheads. There remains a need for test patterns for use in automated alignment of ink-jet printheads which are suited to providing a variety of printhead alignment information in a compact format.